Surface Resonance…

The sound art research program developed from applied research into low frequency sound and sensation perception. I was interested in the way the body processes sensation and the dialogue between the senses.

This section is a reference for discussion of vibration perception within this site. It constitutes the findings of research undertaken before the commencement of Master of Art studies.

This page covers research findings on vibration and vbro-acoustic perception, including vibration and auditory interactions and vibro-acoustic therapy. It also addresses early vibration floor testing exploring envelope, frequency, crossover, and audience engagement and feedback.

The Master of Art candidature developed from a Graduate Certificate and preliminary Masters research at the Spatial Information Architecture Laboratory (SIAL).

The initial focus was on using floor-based vibration as a substitute for subwoofer sound in a nightclub setting, and the noise reduction benefits this would offer. I later moved departments to refocus my work on the creative potential of vibration technology.

This background research included:

• Relating soundscape studies to the experience of low frequency sound in contemporary music culture
• Researching sound therapy, sound perception and engineering / noise studies on low frequency sound and vibration
• Prototype design of a small, low frequency sensation generating floor, and testing with a group of participants

Research findings - vibration and vibroacoustic perception

The purpose of my research in this area was to broadly understand aspects of vibration perception that could inform my approach to composition and installation.

Vibration perception

The processing capacity of the tactile system has been explored in detail through research with an industrial/noise focus, with key literature broadly distinguishing between finger transmitted and whole body vibration.¹

The ‘loudness scale’ for vibration differs to that of sound², i.e. an equal increase in the dB level of vibration and acoustic signal won’t lead to the same perceived increase in level across both senses. The vibration response can be understood to generally be more ‘linear’ with stimulus increase, whereas hearing perception is more like a ‘logarithmic’ function (as equated in the dB scale for sound pressure and audio signal).

Comparison of airborne with structural vibration is an emerging field,³ and research and anecdotes around low frequency sensation from soundwaves cannot be assumed to transfer to tactile/ vibration perception.

Some vibration studies were not directly relevant to the whole body vibration focus of my work, such as those of haptic feedback for fingers or hands or skin effects. However, I concluded that these could still be relevant to understanding the way people process and perceive vibration in a whole body vibration context.⁴ For example:

• With skin vibration, different frequencies carry different subjective effects, such as a buzzing vs a smooth experience
• People have limited capacity to discriminate different frequencies through skin vibration
• Perceptual effects can occur depending on the type and closeness of vibration impulses. For example, depending on the timing of impulses, the vibration experience may be enhanced, or signals may appear to be summed together or suppress or mask one another.

Vibration and auditory interactions

There is substantial frequency-range cross over between the active range of vibration and auditory senses, with hearing extending below 20Hz (with loss of tonal perception), and vibration ‘effective’ at a range of frequencies, depending on its source and the type of body connection (i.e. skin, whole body or finger).

Compared to the auditory sense, the tactile sense is relatively lacking in sophistication.⁵

Studies on perception of timing^{6 7 8} of musical auditory/tactile events found that:

• The auditory and tactile systems have different processing delays before the signal is recognised by the brain, with sensation taking longer to process
• People will discriminate when the vibration and acoustic events are not ‘simultaneous’, and may find this distracting
• The perception of simultaneity is not absolutely precise. Within a tolerance range (measured in milliseconds), sound/ vibration events do not have to be exactly simultaneous to be perceived as such⁹
• The envelope of a sound has a significant effect on the timing requirements for perceived simultaneity,¹⁰ with a more gradual envelope reducing perceptual discrimination of timing offsets
• One study used an interactive touch/ feedback sensation with an acoustic system in a virtual environment. Increased acoustic level made people think the sensation level had increased.¹¹

Vibro-acoustic therapy and tactile systems

Vibro-acoustic therapies offer a range of positive physiological and psychological effects that come from vibration stimulation to the body (usually while listening to music), such as relaxation to muscles, pain relief, assistance with brain disorders and injuries, and other rehabilitation benefits.¹²

Vibro-acoustic therapy tends to use vibration that either reproduces the low-frequency parts of music, or uses pulsed sinusoidal tones (such as from beat frequencies), in line with the principle that ‘exposure to soft, low frequency and non-rhythmic music... results in physiological responses indicative of relaxation.’¹³

Significant levels of high frequencies through a vibro-acoustic system can lead to undesirable or distracting acoustic noise.¹⁴

When used in a vibro-acoustic system, the vibration signal needs to be electronically compressed (dynamics reduction) to perceptually align to the acoustic information.¹⁵

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Vibration floor testing

The vibration floor and soundsystem tests explored how people respond to different frequencies and intensities of low frequency sound and vibration.

In each test, audio material was sent as sound through a large subwoofer and loudspeakers, and then sent through the vibration floor instead of the subwoofer.

I interviewed participants about how the sensation experiences differed, their general observations of the sound and their spatial and bodily connection to it.

Testing used swept (fixed or pulsed) sine tones, and a selection of bass heavy music with contrasting frequency content.

The vibration/acoustic setup and ‘tuning’ applied findings from my journal research. For example, I used:

• Equalisers and crossovers, to reduce unwanted high frequency noise through the floor, and roll off the acoustic level to enable a smooth transition to sensation (crossover point ~70Hz)
• Compression of the vibration floor signal, to match the perceived vibration and acoustic levels
• Delays, to account for timing offsets from the distance from the loudspeakers to the floor, and sensory processing differences.

I refined these parameters through participant feedback, and found that the ideal settings accorded well with what was suggested through other research.

The following test outcomes were the most valuable to the ideas and methods I took to sensation composition during the Masters research.

Comparing modes of presentation

Using the vibration floor did not simply replace the acoustic energy with a more direct sensation interface. The qualities of sensation were fundamentally different, being more grounded in the body.

Overall, at most frequencies participants found that they were more comfortable with vibration energy than with the acoustic only system. However, in the higher bass frequencies acoustic and vibration energy became more similar in sensation qualities.

Vibration sensation ‘bodily placement’ (localised sensation) did not correlate with findings from the acoustic only system, suggesting that the way the body is excited by direct vibration is significantly different. Some effects were specific to the vibration system, such as a sense of the feet shaking. The range of sensations experienced were broad, including effects in the upper body, face, legs and back.

In most tests the acoustic only system was less enjoyable for participants. Because the acoustic energy did not involve the body as directly, low frequency information was less tangible and less defined.

Compared to the vibration floor setting, the sound-only setting led to additional reverberation of low frequency sound within the space. This significantly ‘muddied’ the overall sound experience.

Interesting perceptual responses to sensation

The testing suggested unique perceptual quirks when people experience sensation as vibration. Participants reported the sensation as moving around the body with different frequencies, skin sensation, a sense of vertigo and a feeling of tiredness occurring with specific frequencies, and perceptual placement of frequencies within the room.

However, these responses were not consistent between participants, and there were not specific frequencies with predictable effects. I suspected that testing in open spaces or specially designed testing environments may have provided more consistent results.

Participants engaging with the direct sensation

Most participants quickly found that they became more involved with music when the vibration floor was engaged. In addition to compelling more attention to the music, the floor itself became something to interact with and was missed when deactivated.

Descriptions included: the floor creating a nicer experience because sensation was more bodily, feeling more involved and consumed, increased awareness of high frequency sound (in its relationship to the sensation), increased awareness of low frequency sound, quick association with the vibration experience, and that the floor created another level of interaction or relationship.

Frequency

Each music test track had a different frequency emphasis. Participants favoured pieces with frequency content extending into the very low frequencies (20-45 Hz), where the contribution of the floor was felt to be more essential.

Tracks where frequency emphasis was centred closer to the upper operating frequency of the floor (~85Hz) still benefited. However, vibration effects were noticed as ‘buzzing’ or were perceived to be located in the floor, rather than a logical extension of audible sounds. This may have been partly due to the type of technology used (modified loudspeakers rather than dedicated vibration actuators).

Participants found that when the low frequency information was distinctly different from high frequency information, the floor failed to enhance the music experience. Rather, with auditory and vibration senses processing separate information, the floor became a distraction and could be disorientating due to lack of cohesion.

This was primarily noticed on one track, which had an atypical gap in information between low and high frequencies. All other tracks contained some degree of acoustic ‘artefacts’ in the upper harmonics of the predominant (vibration sensed) bass information, which possibly aided in tying sensory information together.

Envelope

Participants favoured the vibration interface in tracks where low frequency rhythm was not overtly punctuated (i.e. having a rolling or fuzzy quality). More punctuated rhythm appeared to highlight differences between acoustic and vibration information, perhaps because the chronological space of definite ‘beats’ promoted more critical perceptual processing.

This result was consistent with the findings of William Martens on envelope and timing delay tolerances.¹⁶

Dialogue between the senses

Feeling the bass as vibration sensation ‘muted’ auditory sensitivity to the complementary sound. Most participants did not perceive acoustic energy in the room until acoustic levels were much higher than the normal perceptual thresholds.

With sine tests, most of the participants identified the same frequency point (56-57Hz) where the energy shifted from vibration to acoustic borne.

However, while this correlation was notable, I suspected it was influenced by the specifics of the testing room and audio setup. For example, room modes and resonances, and the slightly uneven frequency response of the loudspeakers at the crossover points would have affected the audio levels around the room, depending on the frequency.

One participant identified a frequency where they felt that their focus was flipping between the vibration and acoustic system, at a point where both energies were perceived to be at similar levels. This suggested that, although there is some degree of perceptual blending with combined vibration/acoustic energy, the potential exists for sensory confusion when the two sources of information are similar.

Similarly, tests with music tests tracks suggested that vibration and acoustic perception, although working together, can compete with and affect each other, and this effect is dependent on the frequency make-up of each source. For example, the frequency focus of the bassline, the transition from low to high frequency content, and the type of frequency gaps between the very low and midhigh frequencies will all influence the results.

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<sup>1. Griffin, M. J. (1990). Handbook of human vibration. London ; San Diego, Calif., Academic Press.
2. Griffin, 1990; Verrillo, R. (1992). Vibration sensation in humans. Music Perception 9(3): 281-302.
3. William Martens, personal correspondence.
4. Verrillo, 1992
5. Dalgarno, G. A vibroacoustic couch to improve perception of music by deaf people and for general therapeutic use. 6th International Conference on Music Perception and Cognition. Aug 5th, 2000. Keele University
6. Altinsoy, E., Blauert, J., and Treier, C. (2001) Inter- Modal Effects of Non-Simultaneous Stimulus Presentation, Proceedings of the 17 th International Congress on Acoustics. Rome, Italy
7. Daub, M., and Altinsoy, E. (2004). Audiotactile simultaneity perception of whole-body vibrations produced by musical presentations, in Proceedings of the CFA/DAGA’04.
8. Martens, W. L. (2004). Perceived synchrony in a bimodal display: optimal intermodal delay for coordinated auditory and haptic reproduction. International conference on auditory display, Sydney, Australia.
9. Martens, W. L. (2005). Tolerance for delay between whole-body vibration and audio reproduction of musical sound. Twelfth International Congress on Sound and Vibration, Lisbon.
10. Martens, 2005.
11. Altinsoy, M. E. (2003). Effect of loudness on the haptic force-feedback perception in virtual environments. Journal of the Acoustical Society of America 114(4): 2330.
12. Hooper, J. (2002) Is VA therapy, music therapy? Music Therapy Today (online), available at http:// musictherapyworld.net
13. Hooper, 2002
14. Dalgarno, 2000
15. Dalgarno, 2000
16. Martens, 2005</sup>

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SIAL testing

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